This issue is occurring on 4.4.24.2.
When saving a recurring event with multiple events on the same day, the expected behavior is to auto-generate a unique URL for each instance, such as:
mysite.com/events/the-event-name/2018-04-20
mysite.com/events/the-event-name/2018-04-20/2
mysite.com/events/the-event-name/2018-04-20/3
..etc
This is what occurs when an event is saved for the first time. However, when a new item is added to the series the permalinks for the new instance is the same as the first instance, i.e.:
mysite.com/events/the-event-name/2018-04-20
when it should be:
mysite.com/events/the-event-name/2018-04-20/4
This seems like a bug, in that updating the series should create additional permalinks.